<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
    <style>
    #test{
        width:100px;height:100px;
        background:#ff2d51;
    }
    </style>
</head>
<body>
    <div id="test"></div>
    <script>
     var test=document.getElementById("test");
    //  var bg=getComputedStyle(test).backgroundColor;
    //  console.log(bg);

    var bg;
    try{
        bg=getComputedStyle(test).backgroundColor;
    }catch(err){
        bg=test.currentStyle.backgroundColor;
    }
    alert(bg);
    </script>
</body>
</html>